Amplification of signals frequently involves a trade off between linearity and such factors as power consumption, thermal efficiency, cost, size, etc. Traditional amplifier design achieves linear operation at the expense of power efficiency; class A is a good example of high linearity combined with poor efficiency. Improved power efficiency is usually achieved through the use of class AB, B, and C amplifier designs but at the expense of linearity. However, in recent years new techniques have been developed which improve linearity of these designs using feed-forward cancellation of undesired amplification artifacts (distortion). These techniques enable the design of amplifiers having both good linearity and efficiency and, as such, are particularly useful in high power applications such as the radio tv broadcasting industry and in limited power applications such as satellite transponders. While feed-forward cancellation does provide significant advantages, the need continues for improved amplifier design techniques.
Numerous examples of feed-forward distortion cancellation are taught in the existing body of patents. U.S. Pat. No. 4,532,478 to Silagi (1985) is an early example wherein feed-forward cancellation is achieved via implementation of two loops. The first loop is around the amplifier to be linearized. Its purpose is to create a replica of the distortion generated by the amplifier by subtracting the input signal from a gain and phase normalized version of the distorted and amplified output signal. The second loop adjusts the amplitude and phase of the distortion output of the first loop and then subtracts it from the output of the amplifier. An error amplifier is required in the second loop in order to amplify the distortion to a level where it will be of sufficient amplitude to cancel the high level distortion generated by the main amplifier. Because the two loops in Silagi's patent are manually adjusted, any variations in input signal level and frequency and/or component values due to such factors as temperature and aging will tend to unbalance the loops and adversely effect the degree of distortion cancellation.
Newer feed-forward implementations such as those taught in U.S. Pat. No. 5,148,117 to Talwar (1992) employ methods of automatically balancing the two loops and thus compensating for changes in input signal parameters and environmental conditions. An error amplifier is still required in the second loop. The error amplifier must be highly linear and low noise or it will itself degrade the output signal. Such amplifiers typically operate in the inefficient class A mode to achieve the desired degree of linearity. The presence of the error amplifier can be costly in terms of power consumption, heat dissipation, size and dollars thus negating to some degree the gains achieved by employing feed-forward in the first place.
The need for a high-level error amplifier is eliminated if feedback techniques are employed instead to suppress distortion. It is well known that negative feedback will decrease certain types of distortion; however, the instantaneous bandwidth over which such improvement can be realized is limited by loop gain and the accumulated phase shift of the loop. Stable feedback loop design places stringent requirements on loop phase and gain characteristics.
U.S. Pat. No. 4,276,514 to Huang (1981) discloses a feedback technique in which an amplitude adjusted sample of the amplifier output is subtracted from a phase adjusted sample of the input signal. The resultant is then passed through a bandpass filter and a delay equalizer then summed into the amplifier input via gain and phase adjusting networks. Huang attempts to broaden usable loop bandwidth by incorporation of a SAW delay equalizer in the feedback path. However the general utility of such an approach is limited in that addition of any network inserted into the feedback path will increase time delay and thus narrow usable loop bandwidth. As with Silagi, no provisions are made for automatic adjustment of loop phase or amplitude.
In U.S. Pat. No. 4,929,906 to Voyce and McCandless (1990) a method of achieving linearization of rf amplifiers over a wide loop bandwidth is taught. Use is made of down/up conversion in order to lower the frequency at which the feedback loop operates. The input signal is first down-converted to some convenient lower frequency and then passed to the feedback loop summing junction. It then passes through an IF filter and an up-converter before reaching the amplifier input. A sample of the amplifier output is directly fed back to the loop summing junction via a second down converter. A key feature of the Voyce and McCandless patent lies in that as the summing frequency is lowered, loop implementation is in general simplified. However, forward gain of the amplifier is reduced by the feedback and no provision is made for automatic adjustment of the loop phase shift.